The present invention relates to a pulse receiving system and method for use in the receiver section of a pulse communication system, and more particularly to a pulse receiving system and method using heterodyne detection in an optical wave band or the like.
The optical heterodyne detection technique, by which optical signals are detected by a heterodyne procedure, gains the advantage of permitting substantial improvement in the optical receiver sensitivity over the direct optical detection technique. According to this optical heterodyne detection method, the signal light which is a modulated carrier having a high frequency of hundred IHz, and the local oscillator light having substantially the same frequency are mixed by an optical detector and converted into an electric a1 signal having an intermediate frequency in the VHF band or the microwave band, and then this signal is demodulated into the original electric a1 signal.
Previously, where the demodulated electric a1 signal was a pulse signal, the intermediate frequency had to be set sufficiently high, no less than 15 times as high as the repeating frequency of the pulse signal. There was the disadvantage that, unless the intermediate frequency was high enough, a waveform distortion would arise, with the amplitude, width and phase of the pulse differing from one pulse to another, resulting in a deterioration in the optical receiver sensitivity.
However, a high intermediate frequency would necessitate an intermediate frequency amplifier in the high frequency band. This would entail a greater cost and a higher level of thermal noise, requiring a high level local oscillator. If the level of the local oscillator light were low, the optical receiver sensitivity would deteriorate. Therefore, it is desirable to set the intermediate frequency at a low value without inviting pulse waveform distortion.